The present invention generally relates to channel access systems, and more particularly to a channel access system for a PCM first order terminal equipment.
In a PCM first order terminal equipment which employs a distributed processing in which a signal from a switching system is converted into a PCM signal in each channel, there has been much progress in making the internal structure of the terminal equipment in the form of a large scale integrated circuit (LSI). For this reason, the size of the terminal equipment is reduced, and consequently, there are demands to reduce the line density between a channel part and a multiplexer part.
FIG. 1 shows an example of a conventional channel access system. The channel access system generally comprises n channel parts 11.sub.1 through 11.sub.n and a multiplexer part 12. The n channel parts 11.sub.1 through 11.sub.n have the same structure and only the structure of the channel part 11.sub.1 is shown for the sake of convenience. In addition, FIG. 1 shows only a transmission system of a CEPT (Conference of European Postal and Telecommunication Administrations) system PCM first order terminal equipment for the sake of convenience, but a reception system of the CEPT system PCM first order terminal equipment operates similarly to the transmission system.
The channel part 11.sub.1 comprises a transformer 13, a PCM coder 14, a signaling converter 15, and decoders 16 and 17 which are connected as shown for one channel. FIG. 1 shows only the structure of the channel part 11.sub.1 for one channel, but the channel part 11.sub.1 may include the transformer 13, the PCM coder 14, the signaling converter 15, and the decoders 16 and 17 may be provided for a plurality of channels.
An analog signal from a switching system (not shown) is supplied to the PCM coder 14 via the transformer 13 of the channel part 11.sub.1. On the other hand, the analog signal from the switching system is supplied to the signaling converter 15 via a primary side of the transformer 13. The PCM coder 14 receives a first synchronizing signal SYNC1 from the decoder 16 and converts an input analog voice signal into a PCM signal in synchronism with the first synchronizing signal SYNC1.
The signaling converter 15 converts the analog signal into a signaling data SIG on a PCM line based on a second synchronizing signal SYNC2 which is received from the decoder 17.
The PCM signal which is obtained in the PCM coder 14 is supplied to a voice multiplexer 18 of the multiplexer part 12 as a voice frequency data VF. This voice multiplexer 18 multiplexes the voice frequency data VF with voice frequency data from the other channels. The voice multiplexer 18 also supplies channel access signals to each of the channel parts 11.sub.1 through 11.sub.n. For example, the decoder 16 receives the channel access signals from the voice multiplexer 18 and detects a channel access signal which designates its own channel by decoding the channel access signals. The decoder 16 supplies the first synchronizing signal SYNC1 to the PCM coder 14 whenever the channel access signal for that channel is detected.
A signaling multiplexer 19 of the multiplexer part 12 adds a multiframe pattern to the signaling data SIG which is received from the signaling converter 15 and to signaling data which are received from other channels. The signaling multiplexer 19 then time-division-multiplexes these signaling data based on the timing signals which are received from the voice multiplexer 18. A signaling time division multiplexed signal from the signaling multiplexer 19 is supplied to the voice multiplexer 18. On the other hand, the signaling multiplexer 19 supplies a channel access signal of the signaling system to the decoder 17.
The voice multiplexer 18 time-division-multiplexes the voice frequency data VF and the signaling time division multiplexed signal according to a predetermined rule and generates a digital signal by adding a frame pattern to the time division multiplexed signal. For example, one frame of the digital signal is made up of a total of 32 time slots "0" through "31" each having 8 bits. The frame pattern is arranged in the time slot "0". On the other hand, the multi-frame pattern for the signaling data SIG and the signaling data SIG are arranged in the time slot "16". The voice frequency data VF of each of the channels are arranged in the remaining 30 time slots. In this case, the digital signal having the structure of a 30-channel PCM system is transmitted to the PCM line at a transmission speed of 2.048 Mbit/s.
However, according to the conventional channel access system, the channel access signal of the voice frequency system supplied from the voice multiplexer 18 to the decoder 16 and the channel access signal of the signaling system which is supplied from the signaling multiplexer 19 to the decoder 17 are mutually independent. This is because the transmission speed of the signaling data SIG is considerably slower than the transmission speed of the voice frequency data VF. For example, the transmission speed of the voice frequency data VF is 1.984 Mbit/s while the transmission speed of the signaling data SIG is 64 kbit/s.
For this reason, independent transmission lines are required for the channel access signal of the voice frequency system and the channel access signal of the signaling system. As a result, a large number of lines are required between the channel parts 11.sub.1 through 11.sub.n and the multiplexer part 12 and it is necessary to use connectors having a large number of pins. In addition, even when parts of the channel parts 11.sub.1 through 11.sub.n and the multiplexer part 12 are reduced in size and made in the form of printed circuits, a large number of lines are still required to make the necessary connections. Consequently, the line density of the backboard becomes relatively high and a multi-level printed circuits must be used for the backboard. Therefore, there is a problem in that the channel access system becomes expensive due to the large number of lines which are required to make the necessary connections.